LO 11-3 Identify and explain the purpose of coolant circulation patterns in diesel engines

LO 11-03: Identify and explain the purpose of coolant circulation patterns in diesel engines.

Cooling System Circulation Patterns

Because the operating clearances of an engine are precisely machined to allow for the expansion of hot engine parts, the engine operating temperature in conventional cooling systems is regulated to a narrow operating temperature range. In all liquid-cooled engines, a liquid thermostat controls the flow of coolant through the radiator. The same arrangement is necessary in a hybridized cooling system, which is a conventional cooling system with partial or complete electronic controls, such as the use of an electronically controlled thermostat. In high-capacity, high-flow cooling systems needed to remove a large amount of heat, more than one thermostat is often used. When cold, the coolant does not circulate through the radiator because the thermostat blocks its flow. Instead, coolant circulates through the water pump and flows out through a bypass hose or passageway, recirculating coolant only through the cylinder block and head and back into the water pump inlet. Without the coolant bypass passageway, coolant could neither move at all through the block and cylinder head nor flow through the heater core in the passenger compartment.

Thermostats, which regulate coolant flow, are commonly offset to one side of the engine and are held in their own housing near the cylinder head. When the thermostat opens, hotter, less dense coolant from the cylinder head flows out through a large upper radiator hose into the radiator and returns via the lower radiator hose to the water pump inlet. There are variations, but coolant generally flows through the oil cooler first after leaving the water pump. After passing through the oil cooler, coolant then flows through the cylinder block water jackets from front to rear and at the same time from the side of the cylinder block with the thermostat to the opposite to side of the cylinder block before flowing into the cylinder head, crossing through the head, and then finally passing out through the thermostat again to the radiator. The direction of coolant flow from the block to the head is controlled by coolant holes in the head gaskets. By using different sizes of holes or more holes on one side of the head gasket, a slight pressure differential is created in the coolant pressure within the cylinder head. The largest holes are generally located on the opposite side of the gasket from the thermostat Figure 25. This arrangement enables coolant to flow across the head to properly cool the head before reaching the thermostat housing.

FIGURE 11-25: Coolant flow across the cylinder head and through the block is controlled via coolant pressure differentials created between water pump pressure and differently sized holes in the head gasket.

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